The present invention relates generally to apparatus employed in the continuous strand casting of molten metal, such as steel, and more particularly to a tundish which contains structure for controlling the flow of molten metal therein.
In continuous strand casting, molten metal is poured from a ladle into a tundish having a multiplicity of outlet openings from which exit a multiplicity of molten metal strands each directed into a mold through which the strand moves, and the strand is solidified as it moves through the mold. It is desirable that each of the strands exiting from the tundish be of substantially uniform quality and composition with a minimum of inclusion-type impurities. This requires that the molten metal exiting through each of the outlet openings in the tundish be subjected to substantially the same amount of mixing action in the tundish, have essentially the same residence time in the tundish, and be subjected to a sufficient amount of slagging action to remove inclusion impurities from the molten metal to the extent desired.
Inclusion impurities are removed in a tundish by floating a slag cover on top of the molten metal in the tundish and subjecting the molten metal to a period of contact with the slag cover during which the molten metal is not itself undergoing a mixing action. This can be accomplished by providing so-called "plug flow" to the molten metal, as will be explained below in more detail.
A certain amount of mixing action is desirable, before the molten metal undergoes plug flow to the outlet openings, because this contributes to uniformity in the composition of the metal exiting through the various outlet openings in the tundish.
It is also desirable to minimize the locations in a tundish where there are dead zones of molten metal, that is, locations where there is neither a mixing action nor a plug flow. Dead zones are undesirable because the molten metal at those locations undergoes little or no mixing, and dead zones can result in the formation in the tundish of skulls (solidified volumes of metal).
A tundish of the general type which the present invention is intended to improve is elongated and comprises a pair of side walls disposed substantially in the longitudinal direction of the tundish and a pair of opposite end walls each extending in a lateral direction between the side walls. The tundish has a bottom and a substantially open tundish top. The tundish bottom has a plurality of molten metal outlet openings all of which are aligned in a row extending longitudinally between the tundish end walls. In one kind of tundish, there are a pair of inner outlet openings and a pair of outer outlet openings, and all of the openings in the row of outlet openings are generally equally spaced from each other.
Molten metal is directed from a ladle through a ladle nozzle toward the tundish bottom at a ladle nozzle stream impact location laterally spaced from the row of outlet openings and disposed between the end walls at a substantial distance from each end wall, typically midway therebetween. Molten metal impinges against the ladle bottom at that location and flows from there along the ladle bottom to other areas of the tundish.
Molten metal exits as strands from all of the outlet openings in the tundish bottom and passes into the solidification molds. However, in a tundish of the type described above, the quality of molten metal exiting from the inner pair of outlet openings differs from the quality of molten metal exiting from the outer pair of outlet openings, and this is undesirable. In addition, there is a substantial dead zone volume for the molten metal when employing a tundish of the type described above. Another drawback is that the stream quality for the inner strands is poor in that it displays significant "roping", a form of turbulence in the stream. Roping is undesirable because a stream with roping has more surface area exposed to the surrounding atmosphere than a stream without roping, thereby increasing the stream's susceptibility to oxidation and rendering the molten metal "dirtier" which is undesirable.
The defects described in the preceding paragraph are due to the fact that there is a short-circuiting of molten metal to the inner outlet openings and that there is a relatively low volume fraction of plug flow to the inner outlet openings. Plug flow refers to molten metal (or fluid) which flows as a plug from a location where it has undergone mixing to the outlet opening. This is flow as in a pipe. A volume of molten metal undergoing ideal plug flow does not undergo mixing or have turbulence within itself. As a result, inclusions can be removed from that volume of metal into a slag cover atop the bath of molten metal in the tundish. In a volume of molten metal undergoing mixing action within itself, this cannot occur.
Because the plug flow volume to the inner outlet openings is relatively low, both in an absolute sense and in comparison to the plug flow volume to the outer pair of outlet openings, the slagging out of inclusions from molten metal exiting through the inner pair of outlet openings is both less than desirable and less than occurs in the molten metal exiting through the outer pair of outlet openings.